The present invention relates to data transmission and, in particular, to the channel coding of data using trellis or other redundancy codes.
The co-pending, commonly assigned patent application of W. L. Betts et al filed Jan. 13, 1992, Ser. No. 07/820,111 entitled "Data Communication System with Data Rate Throttling," discloses an advantageous technique, referred to as "data rate throttling," in which the rate--in bits per second--at which data is channel mapped and transmitted over a communication channel can be easily varied in small increments via the use of a so-called modulus converter. The modulus converter itself is disclosed in another co-pending, commonly assigned patent application of W. L. Betts et al entitled "Modulus Converter for Fractional Rate Encoding," Ser. No. 588,658, filed Sep. 26, 1990.
The present invention is directed to a technique which can be used to particular advantage to realize systems which use the aforementioned data rate throttling technique in conjunction with trellis, or other, redundancy coding.
In redundancy coding, generally, the plurality of channel symbols, or signal points, which comprise the so-called symbol constellation are partitioned into a plurality of channel symbol subsets, typically in such a way that the channel symbols of the various subsets are well separated from one another. A portion of the data to be communicated--referred to as the "coded data"--is processed using a redundancy code, such as a trellis code, and the resulting coded output is used to identify a sequence of the subsets. Within a subset, each symbol has an assigned index value, and the rest of the data--referred to as the "uncoded data"--is used to generate an index value for each subset of the sequence, thereby selecting a particular symbol from each subset. A signal representing the sequence of symbols thus selected is then transmitted over the communication channel.